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DURHAM, N.C. -- It could be the fellowship, the ceremonies, or the connection with a higher power, but whatever the reason, religious practices have a beneficial effect on physical and mental health.

More than a dozen studies conducted at Duke University Medical Center have shown how religious activities can improve health, from boosting immune function to speeding recovery from depression.

Now, Duke researchers have discovered yet another health benefit of religious activity: it maintains lower blood pressure.

In a study of 4,000 North Carolinians ages 65 and older, researchers found that the more religious the person is, the lower his blood pressure is. Specifically, they found that people who attended religious services and prayed weekly, or studied the Bible at least once a week, were 40 percent less likely to have high diastolic pressure or diastolic hypertension -- associated with heart attacks and strokes -- than those who didn't participate in either activity regularly. Religious participants also experienced smaller increases in blood pressure over the years than their nonreligious counterparts, researchers found.

Funded by the National Institute on Aging, the study will be published in the August issue of the International Journal of Psychiatry in Medicine.

"The likelihood of this finding happening randomly is less than one in 10 thousand," said Harold Koenig, an associate professor of psychiatry at Duke who co-authored the study. "That finding holds up even after you take age, sex, race, smoking history, and a number of chronic illnesses into account."

While religious activities protected all participants from age- and stress-related increases in blood pressure, this effect was particularly strong in blacks and those under the age of 75. "In these subgroups, you could predict ahead of time what their blood pressure would be during the next wave of the study, based on their religious activity," said Koenig.

The researchers conducted the study in three waves in 1986, 1989 and 1992 – to confirm that religious activity, and not some unrelated variable, was responsible for lower blood pressure levels. During each wave, participants gave a self-evaluation of their religious activities. Researchers also monitored the participants' blood pressure as well as their health status and levels of chronic disability.

"When we analyzed the data, we controlled for health status and chronic disabilities that might prevent people from going to church," Koenig said. "In this way, we were able to rule out the possibility that high blood pressure affected religious participation rather than the other way around." Researchers aren't sure why religious practices have such a positive effect in older people, but Koenig theorized that religious beliefs and participation enable people to cope better with difficult life circumstances. People under the age of 75 seemed to benefit the most in the current study, perhaps because high blood pressure in the 75-plus age group is more related to poor health than it is to psychological or spiritual factors, Koenig said. "Our studies have repeatedly shown that the emotional effects of religious activities have physical consequences throughout the body," Koenig said. "In this case, the effect we found was on blood pressure. If you cope with stress better, your blood pressure isn't going to be as high."

Co-author Linda George, a Duke sociology professor, offered a similar theory.

"Religious people have better support systems which keep them healthier. The sense of meaning and kind of comfort that religious beliefs provide make them more resistant to stresses both physical and social."

However, George said that she and the other researchers could only speculate on why religious practice affects blood pressure. "We really don't know as to the causal mechanism that underlies this phenomenon," she said.